I realize that carbon fibre is strong, but for some reason, having a bike that isn't made out of metal seems like asking for trouble.

My main reason for not liking it is the failure mode. Carbon won't bend like metal, but rather crack and snap in half. This makes me nervous.

Think of it this way. Over time a frame develops weaknesses. In a metal frame, these weaknesses would cause things to start bending. The bending would be noticeable and I would replace the part before it caused and accident.
However carbon doesn't bend, and it will eventually just break. This would most likely cause an accident. I don't want my front fork to just snap as I'm riding down the road.

Is my concern warranted, or am I just unfamiliar with the technology? How often should carbon parts be replaced? What signs of wear should I look for?

Normally I'd say "Just say NO to carbon", bit I suppose it's getting hard to find a medium-quality bike without at least a carbon fork, because it's become so fashionable.
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Daniel R HicksJul 7 '11 at 19:38

Take 2 carbon based tranquilizers and call me in the morning.
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MoabJul 7 '11 at 21:43

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You might want to take a look at the gallery of bike failures - you might be surprised at how many metal component failures occur without much warning. I'm currently nursing a bum shoulder due to total failure in a 4130 (cromoly steel) rack tube - it developed a hairline crack in a location I wasn't really able to inspect and then proceeded to fail quite suddenly. Bending is basically unknown as a fatigue failure mode for bicycle components. Post-crash, maybe, but not from cyclical loads.
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lantiusJul 7 '11 at 23:39

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You'll also want to check out Craig Calfee's carbon whitepaper, which explains in some detail the characteristics of carbon fiber that make it suitable for bicycle design and why some manufacturing techniques using it are significantly better than others.
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lantiusJul 7 '11 at 23:41

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@lantius - Now Kibbee will be nervous on all bikes.
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Neil FeinJul 8 '11 at 4:31

6 Answers
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Watch the Tour de France...you'd be hard pressed to find a bike that is not carbon. Yes pros do get multiple steeds, but remember riding is their full time job. When was the last time one of us put 30-50 hours on a bike in a given week, never mind for 6-8 months! Now think about the wattage these guys put out and hopefully you are getting the idea...that carbon is a proven material.

Do failures happen? Yes, but the frequency is so minimal and when it does occur it is not the instantaneous snap in half you fear. Furthermore, no multi-million dollar corporation would be able to mass produce a product that posed as serious of risk as you suggest.

As far as carbon forks go, I use them on the road, as well as, cross and have never had a failure. Their ability to absorb vibration is unparalleled and I honestly cannot imagine riding a bike without one. I have a stable of bikes and all have carbon except one and that one has a suspension fork. My 29er MTB has a rigid carbon fork that replaced a ti rigid a couple years ago!

Now having said that, I have been riding carbon frames on the road for 5 years and the benefits are unbelievable. I used to have a carbon mountain bike and it was phenomenal. Carbon is not the end all and be all. There are other materials that ride extremely well.

Carbon fiber is stronger, and far less susceptible to fatigue than any other frame material on the market. It can be engineered to have the strength of titanium, the ride quality of steel, and the stiff and powerful performance of aluminum at the same time.

I have only broken 2 frames in my life. Both were aluminum, ridden far beyond the fatigue life of the metal. I have seen carbon break, but usually it is in a major accident. A car crash or something similar.

If a carbon frame cracks from fatigue, it shows small crack in the paint, then splintering, and then it will look like crushed bamboo when it fails fully. The frame will last longer, and you will have more warning of failure on carbon than any other material.

As for why you should choose it, it makes your ride more comfortable, more stable, and more pleasant. It's also lighter, but that is at best a secondary reason.

I always thought that carbon was stronger but only to a point.
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Neil FeinJul 8 '11 at 4:33

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@Neil, that's true. But that point is based on the engineering of the frame, and it's basic construction. In our shop, we have a carbon tubing cross section cut from a frame for demo purposes. It is a round tube, 2mm wide. It looks like a keyring made of carbon. I can place that ring in a vice and compress it to an oval 1/3 of its width, and it will return to its original shape. No other material except ti will do that. And it will do it over and over again. Push it too far, it will break, yes. But that point is well beyond what would have already destroyed a different material frame.
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zenbike♦Jul 8 '11 at 6:25

Ti will do the same thing, but the fatigue life is shorter, and compression point for permanent damage is lower, so it lasts less time.
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zenbike♦Jul 8 '11 at 6:27

@Vorac: That is not accurate. They have a limited 2 year warranty on the Session (carbon) model frames and swing arms. All of their other carbon frames have a lifetime warranty. As to why, you'd need to ask Trek. But I suspect it has something to do with the way the Session is ridden. Being a freeride/DH machine, it's use is far outside "normal". By the way, the Aluminum Session listed has a 3 year warranty, not lifetime.
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zenbike♦Jun 6 '14 at 15:23

From my understanding, carbon fiber is extremely strong within its designed load parameters. That means the directionality of the load, the amount of load, etc. It is only when one goes outside this load design that you start to see failures (other than manufacturing flaws, but that could happen on any bike).

For example, clamping a carbon fiber handlebar using a stem designed for steel/aluminum. The style of clamp is wrong, and can lead to crushing the bar at the clamp. Same applies for almost all clamping surfaces. Most components are not designed to be clamped down on unevenly.

As for the frame and fork, as long as you don't run into a wall, for example, you are probably going to be fine. Doing so would put stress on the fork in the wrong way, plus try to pull the head tube away from the top tube, a stress it is not intended to handle.

Now, to fully disclose: I weigh 375lbs. Carbon fiber bicycles are NOT designed for my weight, and quite frankly that scares the crap out of me. As a result, I ride Surly bikes. Chromoly Steel with high spoke-count wheels to feel safe.

This is kind of my point. I don't ride in a velodrome. Hitting a deep pothole could cause pressure in the wrong direction on the fork. Also, I'm not sponsored, I want to know if I would have to replace components more often.
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KibbeeJul 7 '11 at 20:26

I've known a couple of guys who ran into parked cars and bent their forks. There you at least know the bike's been seriously damaged (especially when the front tire rubs against the down tube), but it would be hard to tell when a carbon fork's been over-stressed.
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Daniel R HicksJul 7 '11 at 21:50

Well, another thing to consider is if you really need the weight savings. If you aren't racing semi-professionally (10-15 times a year?) I would say not to even bother. The weight savings will be negligible, even for longer rides, vs. the price.
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Jack M.Jul 7 '11 at 21:59

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If I needed to save weight that badly I'd go on a diet.
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Daniel R HicksJul 7 '11 at 22:55

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Riding carbon is not about weight. It's about comfort and performance. @Daniel, carbon forks show their damage, just in different ways. Usually as cracking in the clearcoat over the material. It also takes a much higher impact to do damage. Have you owned/ridden a carbon frame in the last 10 years?
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zenbike♦Jul 8 '11 at 6:28

Yes it is true.. After crushing three specialized Enduros, two Other aluminum frames over the last ten yrs, i have yet to break my white Santa Cruz Carbon Blur LT to this day after over a year and a half... 6,000 miles of trail riding... i am sold on Carbon, and Santa Cruz Bikes does the trick on this end. I also own an old 1989 Kestrel EMS carbon road bike, that i like to take on fireroads in the mountains and on paved roads with lots of DH, potholes, hazards, from the summit to the coast, and after many years of abuse and thousands of miles, not problems at all...
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sovJul 21 '14 at 7:51

I disagree with your assertion that a metal bike is better because gives you warning
It is not common that you see a slight bend prior to total failure.
And a sight bend will typically result in injury also
If the fork bends and traps the wheel on the frame you are going down

Brittle failure is when it snaps and plastic deformation is when it bends.

Metals can also suffer from brittle failure - especially aluminum
A high carbon steel (designed for hardness over strength) can have brittle failure
With loading metals fatigue and often with no visual indicator

Titanium is the least brittle followed by steel then aluminum
Titanium is also expensive

Correct a 3 lb carbon frame is not as strong as 6 lb steel frame
But 6 lb steel is not twice a strong